Exhaust aftertreatment systems are used to remove undesirable emissions from the exhaust of fossil fuel powered systems (e.g. diesel engine, gas engines, gas turbines), which may be used to drive, for example, generators, commercial vehicles, machines, ships, and locomotives. Exhaust aftertreatment systems may include a variety of emissions treatment technology, such as diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), selective catalytic reduction catalysts (SCRs), lean NOx traps (LNTs) or other devices used to treat the exhaust.
Selective Catalytic Reduction (SCR) systems provide a method for removing nitrogen oxide (NOx) emissions from fossil fuel powered systems. During SCR, a catalyst facilitates a reaction between a reductant, such as ammonia, and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas. Generally, the reductant is mixed with the exhaust upstream of the SCR catalyst.
SCR may be more effective when a ratio of NO to NO2 in the exhaust supplied to the SCR catalyst is about 50:50. Some engines, however, may produce a flow of exhaust having a NO to NO2 ratio of approximately 95:5. In order to increase the relative amount of NO2 to achieve a NO to NO2 ratio of closer 50:50, a diesel oxidation catalyst (DOC) may be located upstream of the SCR catalyst to convert NO to NO2. DOCs are also used to remove carbon monoxide and hydrocarbons from the exhaust.
Exhaust aftertreatment systems may be installed as original equipment or may be retrofitted to a specific application. To facilitate easier installation, some exhaust aftertreatment systems are preassembled with components enclosed within a common housing. U.S. Published Patent Application No. 2008/0314033, by Aneja et al. (hereinafter the '033 application), discloses such a system. The '748 application discloses a generally cubical common housing that encloses a pair of DOC/DPFs, a reducing agent injector, a mixing chamber, and a pair of SCR catalysts. Exhaust entering the system is split into two flow streams that are directed through the two DOC/DPFs in parallel. The exhaust is recombined into a single stream to which the reducing agent is injected and is split again into two streams that flow through the two SCR catalysts in parallel.
While the system disclosed in the '033 application may be suitable to reduce target emissions in some applications, specific size and backpressure constraints, specific emissions requirements and other requirements may make it unsuitable for other applications.